Under extrusion after retracts and dimensional errors on Delta

I have now finally gotten to make a few small prints on my DYI delta printer https://www.duet3d.com/forum/thread.php?id=4000
At the moment my setup is:
Bondtech QR extruder
40 cm Capricorn PTFE tubing
E3D Chimerra - 0.4 mm nozzle (So far only using one of the nozzles)

I must say, that i am generally very happy with the looks of my prints and i believe this printer can become very good! Here my third print:

However a few things is still to be addressed.

1. A 20 x 20 x 20 cube measures 19.8 x 19.9 x 20 (x,y,z). I have done 6-factor auto calibrations and are down to 0.016 deviations when i run a G32. Can this dimensional error be related to a delta arm value that is to low? If i think of it, i would rather think that to low a value would create a print that was to large and not to small. Am i wrong?

2. My other issue is something that have been haunting me for all my 3D printing adventures, both with my trusty old Hephestos Prusa with a cheap E3D clone j-jhead to a expensive Core XY with E3D V6 and dyzend.
I always get under extrusion after retracts.. (Pressure advance mittigates this effect a little but it is there with and without) Here is an example of my issue:
This part looks absolutely amazing.. until the thin section where retractions start to really happen. It is easy to see that when retractions happens for the first time, i get under extrusion.
This example print was printed at:
40 mm/s
200 degree nozzle
55 degree bed
ColorFabb black PLA
1.8 mm of retraction at 30 mm/s
0.2 mm layer height

Does anyone know what may be causing my problems? (I know there is a peremiter gap, but that is due to mismatch of extrusion width and model thickness)

1. It's better to use a larger test cube to check dimensional accuracy, to reduce the effect of any under or over extrusion. If after that you still find the dimensions a little under, then print a grid pattern over the whole bed to check linearity. If the grid lines are parallel and evenly spaced, don't adjust the configured arm lengths, instead use M579 to adjust the scaling.

2. How much pressure advance are you using?

If you are getting consistent under-extrusion after retraction, then the "extra length on restart" parameter in the slicer may help you, although I've never needed it.

2. I think this is the pressure advance:
M572 D0 S0.05
So not a lot i think.

It is not always when i retract it seems to be a little random. And adding the extra restart length gave issues on my other printers. Haven't tried it on this one. I thin k i might have missed some fundamental knowledge about 3D printing because i always had this issue to some extend.
On the Core XY i mentioned it looked like random under extrusions and then sometimes over extrusions. This is a picture from testing back then. I do not remember much of the settings though. But it was also short retractions around 2 mm.

Back then i thought i retracted to much so sometimes it would slightly jam and then suddenly over extrude as the jam cleared. Buuut i am not so sure anymore. I was using the minimal amount of retraction to avoid stringing.

When printing the bottom of the pyramid everything looks normal.
As soon as the small sections start all shit breaks loose.. I can see that it underextrudes when it begins on a tower, then suddenly the next tower overflows extremely and so on. I sometimes hear the extruder grinding (i have a very low tension on the QR arm of the bondtech - tightening the screw removes grinding but changes nothing on the print)

S3D is set to relative extrusions, so is the config.g
Temperature seems to be correct-ish. 180 gives grinding when just extruding into the air. 190 -220 works. 230-240 works but clearly to liquid 250 starts to smell burned.

Okei, not much time for testing, but managed to do one small test today. I wouldn't say i got any wiser on the issue but i have hope that it has nothing to do with my printer (hardware or firmware).

The black one you know (disregard that it has a misshapen bottom - i got impatient and ripped it off the hot bed..)
The white one is the one i printed today (remember, no retractions whatsoever):

The only thing i did was: Turn on printer. Heat the nozzle to 200 degrees. Remove the black ColorFabb filament(not even a "Atomic pull"). Install a 5 year old white makerbot leftover. Then just re-print the g-code file already uploaded to the printer.

… Now this strongly suggests that the issue lies with the filament. However, i did not considder ColorFabb to be a bad brand at all! Also it does not explain why the black cube printed fine.. It seems like the black filament prints fine as long as the print has a more or less constant filament flow. As soon as the filament flow pauses i get issues..

My next test will be to try again with the black filament on a cold machine like today. This way i can eleminate heatcreep in my hotend as the issue. I do not belive that is the issue however as i use a 40 mm powerfull fan instead of the stock 35 mm E3D fan. Additionally the cube was printed right AFTER the pyramid without the printer cooling down in between.

I am happy if it is just a filament issue, but i just cant belive it. Either way, I still have absolutely no theory of why the black cube and pyramid are so different (same filament after all)

Then how does that explain that the small cube i showed above is printed fine?
It is printed at exactly the same settings and with the same filament as the pyramid.

When it prints the pyramid it looks like there is a short delay from print move starts til plastic is extruded, then suddenly a big pile of filament is extruded. Can this be caused due to too high temeprature? Also, extruding into free air at the given temperature seems to work fine..

With large parts you can get away with excessively high temperatures or insufficient cooling because when it prints perimeters, by the time it finishes the first one and starts the second, the first perimeter has cooled sufficiently to become solid. That's why the base of the pyramid is ok. What's happening at the sides is that when the second perimeter is printed, the first one is still molten, so it is being pushed out of place by the second one.

Solutions:

1. If using slic3r, set a low speed for "small perimeters". Slic3r has had this setting for at least 4 years, but sadly the other slicers still don't provide it.